1. Field of of the Ivention
This invention relates to improvements in methods and circuits for driving DC brushless, Hall-less, polyphase motors, such as a spindle motor of a hard disk drive, or the like, and more particularly to improvements in such driving methods and circuits that at least reduce the acoustic noise in motors of this type using driving voltages having substantially sinusoidal waveforms.
2. Relevant Background
Efficient motor drive requires that the excitation current in the three motor phases be aligned with the bemf generated by the three phases. One of the best schemes for achieving this alignment is the use of a phase-locked loop (PLL). The phase-locked loop adjusts the phase and frequency of the commutation so the bemf of the un-driven windings passes through zero in the center of the appropriate commutation state. This scheme works well when the shape of the commutation waveforms includes an un-driven region, as in a conventional 6-state, +1, +1, 0, -1, -1, 0, sequence.
Since the +1, +1, 0, -1, -1, 0 sequence has sharp transitions between driving states, this sequence has many high frequency components. These tend to excite mechanical resonances in the motor, which results in the creation of undesirable acoustic noise. Moreover, the step-function tristating of the undriven motor phases, together with the step-function driving waveform produces a degree of torque ripple in the motor. The torque ripple results in an unevenness or jerkiness in the motor rotation, which also excites resonances in the motor, also causing undesirable acoustic noise.
Thus, if it is desired to reduce acoustic noise, a sine wave shaped excitation signal is more appropriate than the 6-state sequence. If the motor driver consists of sinusoidal current sources, the same voltage sensing PLL described above can be used. However, when the duty cycle of the driver is varied sinusoidally, the motor driver excitation is pulse-width modulated (PWM) to minimize power dissipation in the driver IC. This permits lower cost packaging and an overall saving in system cost.
In sine wave excited systems, in the past, in order to estimate the position of the motor, the drive voltage was caused to lead the current by a predetermined amount to compensate for the inductance in the motor windings. Thus, the goal was to achieve a zero crossing of the current simultaneously with the zero crossing of the bemf. It was, however, observed that the actual phase lead is proportional to the magnitude of the current that results from the particular drive voltage that is applied. However, it is difficult to generate currents that have a pure sinusoidal waveform, particularly when the currents are relatively high, and also when a PWM scheme is desired to be used.
To address this difficulty, a small sense resistor was inserted into each drive current leg, and a current sensing loop was used to adjust the duty cycle of the drive voltage. The sense resistors were generally externally supplied by the customer, and their value had to be critically determined. Such precision resistors are relatively expensive, and their effective resistance values were difficult to determine.
In the case of sinusoidal PWM drive, the windings of the motor are alternately connected to the positive and negative supplies. Thus, the winding voltages contain no information about bemf voltage, and a voltage sensing phase detector will not work.
There has been recent emphasis on disk drive manufacturers to reduce the noise associated with disk drive motors. Consequently, what is needed is a disk drive and method for operating it in which the noise associated with the drive in operation is reduced or eliminated. What is additionally needed is a disk drive and method that employs sinusoidal drive signals, or the like, that does not require external sense resistors to determine an estimate of the drive current applied to the motor windings.